1. Field of the Invention
The invention is directed to a method for the preparation of nanoparticles comprising cerium oxide and zirconium. In particular, the invention is directed to a method for the preparation of nanoparticles comprising cerium oxide and zirconium having a narrow size distribution.
2. Background Information
Cerium oxide in the form of fine particles is useful as a catalyst for polymerization, for reforming fuels, and for abating polluting gas in automobile exhaust system. The catalyst acts as an oxygen pressure regulator in the reduction of NOx to molecular nitrogen, the oxidation of hydrocarbons and carbon monoxide to water and carbon dioxide, and the conversion of H2S to H2 and S.
Cerium oxide has been used as a catalyst-component for the recombination of hydrogen and oxygen to water in sealed car batteries, for purposes of extending battery life. Cerium oxide is a good ionic conductor and has been used as an electrolyte material of solid oxide fuel cells and gas sensors.
Cerium oxide has high dielectric constant and a high refractive index making the material suitable for optical coatings, as discussed, for example, in Kanakaraju, S., Mohan, S. and Sood, A. K., Thin Solid Films, Vol. 305, Nos. 1-2 (1997), p. 191. Cerium oxide is also of interest as a catalyst in vehicle emissions systems, as discussed in Trovarelli, A., Boaro, M., Rocchini, E., de Leitenburg, C., and Dolcetti, G., Journal of Alloys and Compounds, Vol. 323-324 (2001), p. 584, and has also found use as a solid oxide fuel cell electrolyte material, as reported in Steele, B. C. H. and Heinzel, A., Nature, Vol. 414, No. 6861 (2001), p. 345; in gas sensors, as described in Stefanik, T. S. and Tuller, H. L., Journal of the European Ceramic Society, Vol. 21, Nos. 10-11 (2001), p. 1967; in high-Tc superconductor structures, as discussed in Walkenhorst, A., Schmitt, M., Adrian, H. and Petersen, K., Applied Physics Letters, Vol. 64, No. 14 (1994), p. 1871; and silicon-on-insulator structures and high storage capacitor devices, as described by Tye, L., El-Masry, N. A., Chikyow, T., Mclarty, P. and Bedair, S. M. Applied Physics Letters, Vol. 65, No. 25 (1994), p. 1030. Because of the relative hardness of the material, cerium oxide nanoparticles are also useful as an abrasive for fine polishing of surfaces of certain materials, such as quartz and silicon.
Some applications may benefit from using monodispersed cerium oxide nanoparticles, due to the possibility of new properties of cerium oxide in the nanodimension. A method and apparatus for the preparation of monodispersed cerium oxide nanoparticles has been described in International Patent Application No. PCT/US02/14539, herein incorporated by reference in its entirety. The small size of cerium oxide nanoparticles is also advantageous because they provide a relatively large surface area, which increases the oxygen storage capacity of cerium oxide. However, the ability of the nanoparticles to store oxygen decreases at high temperatures, such as the temperatures encountered in automotive exhaust systems. This decrease is due to sintering of the nanoparticles at high temperature, which causes at least some nanoparticles to join to form larger particles. As a result of the formation of larger nanoparticles, the overall surface area available decreases.
Nanoparticles of cerium oxide which contain zirconium show increased stability to changes in size upon heating or sintering at high temperatures while retaining all of the beneficial properties and uses of pure cerium oxide nanoparticles discussed above. Such nanoparticles are not only a more thermally stable catalyst than nanoparticles of pure cerium oxide, but also a more effective catalyst than nanoparticles of pure cerium oxide in three-way catalysis and water-gas-shift. The effect of zirconium has been discussed by Mamontov, E., Egami, T., Brezny, R., Koranne, M., and Tyagi, S., J. Phys. Chem. B, Vol. 104, No. 47 (2000), p. 11110, who suggested that the smaller ionic radius of Zr4+ (0.84 Å) relative to Ce4+ (0.97 Å) may promote the formation of Ce3+ ions, which may cause the formation of oxygen vacancies. These vacancies enhance the reactivity of the particles of cerium oxide containing zirconium as a catalyst, as an electrolyte for solid oxide fuel cells, and as a gas sensor.
Several methods have been described for preparing particles of cerium oxide containing zirconium. One approach involves sintering of a mixture of powders of zirconium oxide and cerium oxide above 1400° C. has been described in Fornasiero, P., Monte, R., Di, G., Rao, R., Kaspar, J., Meriani, S., Trovarelli, A. and Graziani, M., Journal of Catalysis, Vol. 151, No. 1 (1995), pp. 168-177, and in Yashima, M., Takashina, H., Kakihana, M. and Yoshimura, M., Journal of the American Ceramic Society, Vol. 77, No. 7 (1994), pp. 1869-74. Both the Fornasiero et al. and the Yashima et al. methods require a very high sintering temperature and produce particles with a large particle size and a very large particle size distribution. Another method involves heating an aqueous mixture of (NH4)2Ce(NO3)6 and ZrOCl2.8H2O having a total molar concentration of zirconium ions and cerium ions of 0.005 M at 100° C. for 168 hours, followed by high temperature sintering of the precipitate, as described in Hirano, M., Miwa, T., and Inagaki, M., Journal of Solid State Chemistry, Vol. 158, No. 1 (2001), pp. 112-17. This method involves a very long reaction time and gives a low yield of the nanoparticles. A further approach involves mixing urea, (NH4)2Ce(NO3)6 and ZrOCl2.8H2O at 100° C. to obtain a gel, boiling the gel for 8 h at 100° C., aging for a period of several days, and sintering the resulting mixture at 650° C., as described in Kundacovic, Lj. and Flytzani-Stephanopoulos, M., Journal of Catalysis, Vol. 179, No. 1 (1998), p. 203. This approach requires a long period of time for processing the gel and gives low particle yields. All methods described above require temperatures of at least 100° C.
Accordingly, a need exists in the art for an efficient method for preparing significant quantities of nanoparticles comprising cerium oxide and zirconium with a relatively narrow size distribution.